Method for bonding a tantalum structure to a cobalt-alloy substrate

ABSTRACT

A method for bonding a porous tantalum structure to a substrate is provided. The method comprises providing a substrate comprising cobalt or a cobalt-chromium alloy; an interlayer consisting essentially of at least one of hafnium, manganese, niobium, palladium, zirconium, titanium, or alloys or combinations thereof; and a porous tantalum structure. Heat and pressure are applied to the substrate, the interlayer, and the porous tantalum structure to achieve solid-state diffusion between the substrate and the interlayer and between the interlayer and the porous tantalum structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional patent application of U.S. patent application Ser. No. 11/870,205, filed Oct. 10, 2007, which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to orthopedic implants, and more particularly relates to a method for bonding a porous tantalum structure to cobalt or a cobalt-alloy orthopedic implant.

BACKGROUND OF THE INVENTION

Orthopedic implants are often utilized to help their recipients recover from injury or disease. To promote quick recovery, orthopedic implants are designed to cooperate with the body's natural inclination to heal itself. Some orthopedic implants are designed to foster osseointegration. As is known in the art, osseointegration is the integration of living bone within a man-made material, usually a porous structure. Cells in the recipient form new bone within the pores of the porous structure. Thus, the porous structure and the bone tissue become intermingled as the bone grows into the pores. Accordingly, orthopedic implants may include a porous surface to enhance fixation between the orthopedic implant and adjacent tissue. Of course, the faster the surrounding tissue grows into the porous surface, the sooner the patient may begin to resume normal activities. However, the manufacture of the orthopedic implants with porous structures is not without difficulty,

Orthopedic implants are usually made from various metals. One difficulty encountered during manufacturing is bonding separate components, each made of a different metal, together. For example, cobalt is a popular metal used to make orthopedic implants, and other popular metals include alloys of cobalt with other metals, such as chromium. The porous structure may be made from an entirely different metal, such as tantalum. In this case, bonding the porous metal to the orthopedic implant involves bonding tantalum to cobalt or to cobalt-chromium alloys. Bonding these two metals together has proved to be particularly problematic.

Thus, there is a need for an improved method of bonding of porous structures, specifically tantalum, to cobalt and cobalt-alloy implants such that the bond has sufficient strength while the corrosion resistance of the metals in the resulting implant are maintained.

SUMMARY OF THE INVENTION

The present invention provides a method for bonding a porous tantalum structure to a substrate. In one embodiment, the method comprises providing (i) a substrate comprising cobalt or a cobalt-chromium alloy; (ii) an interlayer consisting essentially of at least one of hafnium, manganese, niobium, palladium, zirconium, titanium, or alloys or combinations thereof; and (iii) a porous tantalum structure, and applying heat and pressure for a time sufficient to achieve solid-state diffusion between the substrate and the interlayer and solid-state diffusion between the interlayer and the porous tantalum structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

FIG. 1 depicts a cross-sectional view of one embodiment of an assembly comprising a porous tantalum structure, a pre-formed sheet interlayer, and a substrate;

FIG. 2 depicts a cross-sectional view of another embodiment of an assembly comprising a porous tantalum structure, a coating interlayer, and a substrate; and

FIGS. 3 and 4 are photomicrographs corresponding to the embodiments of FIGS. 1 and 2, respectively, following heating and pressing the assembly to bond the porous tantalum structure to the interlayer and the interlayer to the substrate.

DETAILED DESCRIPTION

In accordance with the present invention and with reference to FIGS. 1 and 2, a method for bonding a porous tantalum structure 10 to a substrate 12 generally begins by constructing an assembly 14 comprising an interlayer 16 placed on the surface of the substrate 12 and the porous tantalum structure 10 placed onto the interlayer 16. It will be appreciated that the assembly 14 may be constructed by placing the individual components 10, 12, 16 together in any order that results in the interlayer 16 positioned between and in contact with the substrate 12, and the porous tantalum structure 10, as shown in FIGS. 1 and 2. In other words, the placement order is not limited to those orders described herein.

The porous tantalum structure 10 may be TRABECULAR METAL®, available from Zimmer Inc., Warsaw, Ind. The porous tantalum structure 10 is configured to facilitate osseointegration. The porous tantalum structure 10 may have a pore size, pore continuity, and other features for facilitating bone tissue growth into the pores, as is known in the art.

The substrate 12 may be a cast or a wrought cobalt or cobalt chromium alloy fabricated in a shape according to the requirements for the specific orthopedic application. For example, the substrate 12 may be cast of cobalt in the shape of a total hip replacement implant. Other implants may include implants for the ankle, elbow, shoulder, knee, wrist, finger, and toe joints or other portions of the body that may benefit from a substrate 12 having a porous tantalum structure 10 bonded thereto.

With no intent to be bound by theory, tantalum and cobalt metals are not readily soluble, that is, the documented solid solubility of tantalum into cobalt is insufficient to form the necessary bond strength demanded by applications within the human body. In fact, certain stoichiometries of tantalum with cobalt may prevent solid-state diffusion of tantalum into cobalt and vice versa. Therefore, in accordance with the method of the present disclosure, the interlayer 16 comprises a metal that readily forms solid solutions with both tantalum and cobalt or cobalt-chromium alloys. For example, the interlayer 16 may be any one or an alloy of metals, such as, hafnium, manganese, niobium, palladium, zirconium, titanium, or other metals or alloys that exhibit solid solubility with tantalum at temperatures less than the melting temperature of the substrate 12, the interlayer 16, or the porous tantalum structure 10.

The assembly 14, as shown in FIGS. 1 and 2, may be put together by applying the interlayer 16 to the substrate 12. One skilled in the art will observe that the interlayer 16 may require pre-shaping to improve the contact area between the surface of the substrate 12 and the surface of interlayer 16 prior to applying the interlayer 16 to the substrate 12. Alternatively, the interlayer 16 may be press formed onto the substrate 12 such that the interlayer 16 conforms to the surface of the substrate 12. The surfaces of all components 10, 12, 16 may be cleaned prior to assembly 14 to reduce corrosion and improve solid-state diffusion bonding.

With continued reference to FIGS. 1 and 2, following application of the interlayer 16 to the substrate 12, the porous tantalum structure 10 may be placed on the interlayer 16 thus forming the assembly 14. Similar to pre-shaping the interlayer 16 to conform to the substrate 12, the porous tantalum structure 10 may be formed in a shape to maximize surface-to-surface contact to facilitate solid-state diffusion with the interlayer 16.

Heat and pressure are applied to the assembly 14 sufficient for solid-state diffusion to take place between the substrate 12 and the interlayer 16 and between the interlayer 16 and the porous tantalum structure 10. As is known to those skilled in the art, solid-state diffusion is the movement and transport of atoms in solid phases. Solid-state diffusion bonding forms a monolithic joint through formation of bonds at an atomic level due to transport of atoms between two or more metal surfaces. Heat and pressure may be supplied to the assembly 14 with a variety of methods known in the art. For example, the assembly 14 may be heated electrically, radiantly, optically, by induction, by combustion, by microwave, or other means known in the art. Pressure may be applied mechanically by clamping the assembly 14 together prior to insertion of the assembly 14 into a furnace, or pressure may be applied via a hot pressing system capable of applying pressure once the assembly 14 reaches a target temperature, as is known in the art. Furthermore, hot pressing may include hot isostatic pressing, also known in the art.

Referring now to FIG. 1, in one embodiment, the interlayer 16 is a pre-formed sheet of commercially pure titanium at least about 0.016 inches (about 0.04064 centimeter) thick. In another embodiment, the pre-formed sheet of commercially pure titanium is at least about 0.020 inches (about 0.0508 centimeter) thick for improved bond strength. It will be observed that the interlayer 16 may be positioned directly beneath the porous tantalum structure 10. In other words, it is not necessary to cover the entire substrate 12 with the interlayer 16 to bond the porous tantalum structure 10 at a single location. Furthermore, it will also be observed that the corrosion resistance and the strength of the substrate 12 are not negatively impacted if the porous tantalum structure 10 touches those areas not covered by the interlayer 16 during heating. Thus, the porous tantalum structure 10 may be bonded to multiple separate areas on the surface of the substrate 12 with multiple separate areas of interlayer 16. One skilled in the art will appreciate that the position of the porous tantalum structure 10 may be dictated by the patient's physiological requirements.

In one embodiment, the assembly 14 is clamped together by applying a pressure of at least approximately 200 pounds per square inch (psi) (approximately 1.38 MPa). However, pressures greater than approximately 200 psi may be applied up to the compressive yield strength of the any of the substrate 12, the interlayer 16, or the porous tantalum structure 10. Ordinarily, the porous tantalum structure 10 has the lowest compressive yield strength, for example, 5,800 psi for TRABECULAR METAL®.

The clamped assembly 14 is then heated to at least about 540° C. (about 1004 degree Fahrenheit) in vacuum or in another sub-atmospheric pressure of an inert atmosphere. In any case, the clamped assembly 14 is heated to less than the melting temperature of any of the components 10, 12, 16 and, in most cases, is at least about 800° C. (about 1472 degree Fahrenheit) but less than about 1000° C. (about 1832 degree Fahrenheit) in vacuum. One skilled in the art will observe that the higher the temperature, the less time it will take to achieve solid-state diffusion bonding. The time required to achieve solid-state diffusion bonding may be as little as less than 1 hour to as long as 48 hours and will depend on the metals involved, the temperatures, atmosphere, and the pressures applied.

Once heated to temperature, and after a time sufficient to achieve solid-state diffusion between the porous tantalum structure 10 and the interlayer 16 and between the interlayer 16 and the substrate 12, a construct is formed. The construct may comprise the substrate 12 bonded to the interlayer 16 and the interlayer 16 bonded to the porous tantalum structure 10. FIG. 3 is a photomicrograph of a portion of the construct formed according to one embodiment of the method, described above, with a porous tantalum structure 10 (top) bonded to a titanium sheet interlayer 16 (middle) bonded to a cobalt-chromium substrate 12 (bottom).

With reference now to FIG. 2, in another embodiment, the interlayer 16 is a coating applied to the surface by, for example, thermal spray, plasma spray, electron beam deposition, laser deposition, cold spray, or other method of forming the coatings on a substrate 12. In one exemplary embodiment, the coating interlayer 16 is applied via vacuum plasma spraying, as is known in the art. The substrate 12 may be masked and then grit blasted to prepare the surface of the substrate 12 for vacuum plasma spraying. In one exemplary embodiment, the substrate 12 is masked and then grit blasted with alumina (aluminum oxide) grit for increased corrosion resistance of the construct subsequent to bonding with the interlayer 16. In another exemplary embodiment, the coating interlayer 16 comprises titanium sprayed to a thickness of at least about 0.010 inches (about 0.0254 centimeter) thick. In another embodiment, for increased bond strength, the titanium coating interlayer 16 is at least about 0.020 inches (about 0.0508 centimeter) thick. In the vacuum plasma sprayed embodiments, a porosity level is between about 20% and about 40% for ease of vacuum plasma spray processing while maintaining sufficient corrosion resistance. FIG. 4 is a photomicrograph of a portion of a construct formed according to one embodiment of the method described above, showing a portion of a construct comprising a porous tantalum structure 10 (top) bonded to a titanium vacuum plasma sprayed interlayer 16 (middle) bonded to a cobalt-chromium substrate 12 (bottom).

In one exemplary embodiment, a construct comprising a porous tantalum structure 10 of TRABECULAR METAL® bonded to a titanium interlayer 16 bonded to a cobalt-chromium substrate 12 was characterized by tensile strength testing. Nearly all failure separations occurred in the porous tantalum structure 10. Tensile stresses measured at separation on constructs formed according to the previously described embodiments were routinely above 2,900 psi.

One skilled in the art will observe that heating and applying pressure may include multiple heating and pressurizing processes. For example, the porous tantalum structure 10 may be assembled with the interlayer 16 and bonded thereto, according to one embodiment of the method, to form a subassembly. That subassembly may then be bonded to the substrate 12 according to another embodiment of the method. The reverse procedure may also be used. That is, the interlayer 16 may be bonded to the substrate 12 to form a subassembly with subsequent bonding of the porous tantalum structure 10 to the interlayer portion of the subassembly. Therefore, embodiments of the method may account for different diffusion coefficients between the components 10, 12, 16 which may allow for more consistent, higher strength bonds between the substrate 12 and interlayer 16 and between the interlayer 16 and the porous tantalum structure 10. By way of further example and not limitation, diffusion bonding of a titanium interlayer 16 to a cobalt-chromium substrate 12 at an elevated temperature and pressure may take longer than diffusion bonding of the titanium interlayer 16 to a porous tantalum structure 10 at similar pressures and temperatures. Thus, by diffusion bonding the titanium interlayer 16 to the cobalt-chromium substrate 12 to form a subassembly and then diffusion bonding the porous tantalum structure 10 to the subassembly, a diffusion bond depth between the titanium interlayer 16 and the cobalt-chromium substrate 12 may be substantially the same as a diffusion bond depth between the titanium interlayer 16 and the porous tantalum structure 10. In contrast, if the porous tantalum structure 10, the titanium interlayer 16, and the cobalt-chromium substrate 12 are bonded with a single application of heat and pressure, the diffusion bond depths between the titanium interlayer 16 and the porous tantalum structure 10 and between the titanium interlayer 16 and the cobalt-chromium substrate 12 may be different.

While the present invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept. 

What is claimed is:
 1. A method for bonding a porous tantalum structure to a substrate, comprising: providing a substrate comprising cobalt or a cobalt-chromium alloy; providing a porous tantalum structure; applying an interlayer consisting essentially of at least one of hafnium, manganese, niobium, palladium, zirconium, titanium, or alloys or combinations thereof to a surface of the porous tantalum structure, wherein the interlayer is applied to the surface by at least one of thermal spraying, plasma spraying, electron beam deposition, laser deposition, chemical vapor deposition, or electrodeposition; positioning the substrate in contact with the interlayer thereby forming an assembly, wherein the interlayer is between the substrate and the porous tantalum structure; and applying heat and pressure to the assembly for a time sufficient to concurrently achieve solid-state diffusion between the substrate and the interlayer and solid-state diffusion between the interlayer and the porous tantalum structure.
 2. The method of claim 1, wherein the interlayer is at least about 0.020 inches thick.
 3. The method of claim 1, wherein the interlayer is at least about 0.010 inches thick.
 4. The method of claim 1, wherein the interlayer has a porosity of between about 20% and about 40%.
 5. The method of claim 1, wherein applying the interlayer includes plasma spraying the interlayer onto the surface in at least a partial vacuum.
 6. The method of claim 1, wherein applying pressure includes applying at least approximately 200 psi to the assembly.
 7. The method of claim 1, wherein said interlayer has a top surface and a bottom surface, and wherein said applying heat and pressure to the assembly achieves solid-state diffusion between the substrate and the bottom surface of the interlayer and solid-state diffusion between the top surface of the interlayer and the porous tantalum structure.
 8. The method of claim 1, wherein applying heat includes heating the assembly to less than about 1000° C. in a vacuum environment.
 9. The method of claim 1, wherein the interlayer has a thickness of at least about 0.010 inches and applying heat and pressure includes applying a pressure of at least about 200 psi and heating the assembly to at least about 540° C. for at least one hour to achieve solid-state diffusion between the substrate and the interlayer and between the interlayer and the porous tantalum structure. 